(1.) Field of the Invention
The present invention relates to an apparatus for controlling the speed of an automobile, which apparatus automatically adjusts the speed of the automobile to a preset speed.
(2.) Description of the Prior Art
An apparatus for adjusting the speed of an automobile so that the automobile runs at a constant speed so works that when a desired speed is preset, the degree of opening of the throttle is automatically controlled so that the car constantly runs at the preset speed and thereby, liberates the driver from the task of depressing the accelerator. However, when an automobile provided with such an apparatus is running on an uphill grade, the speed may often gradually decrease despite the fact that the throttle valve is wide. Therefore, the apparatus is equipped with a function which shifts the gears from fourth into third and then back into fourth when the speed reaches the preset speed.
FIG. 1 is a block diagram illustrating major portions of a conventional apparatus for adjusting the speed of an automobile so that the automobile runs at a constant speed. The signal S.sub.V is a signal having a frequency proportional to the speed of the automobile and is converted by a frequency-to-voltage converter 1 into voltages V.sub.SP which are proportional to the speed of the automobile and which are supplied to a subtraction circuit 2. Further, when a desired speed is preset by a speed-setting switch (not shown), a speed-setting unit 3 produces a voltage proportional to the preset speed, and the subtraction circuit 2 generates a differential voltage V.sub.D proportional to the difference between the voltage of the preset speed and the voltage of the actual speed.
The automatic drive control circuit 4 so controls the degree of opening of the throttle valve (not shown) and the like that the differential voltage V.sub.D becomes zero, with the result that the automobile runs at the preset speed. A comparator 5 compares a reference voltage V.sub.R1, generated by a reference voltage source 10, corresponding to a difference in speed of 10 Km/h with the differential voltage V.sub.D generated by the subtraction circuit 2 and sets a flip-flop circuit 7 when difference in voltage is large, i.e., when the speed of the automobile is 10 Km/h less than the preset speed. A comparator 6 compares a reference voltage V.sub.R2, generated by a reference voltage source 11, corresponding to a difference in speed of 4 Km/h with the differential voltage V.sub.D generated by the subtraction circuit 2 and resets the flip-flop circuit 7 when the difference in voltage is small, i.e., when the speed of the automobile approaches the preset speed; in other words, when the speed of the automobile is 4 Km/h less than the preset speed.
The output from the flip-flop circuit 7 is amplified by an amplifier 8 and is supplied to an overdrive-cutting solenoid 9 to energize the overdrive-cutting solenoid 9. The overdrive-cutting solenoid 9 shifts gears from overdrive, e.g., from fourth into third when it is energized. When an automobile is in overdrive, the rotation speed of the engine shaft is less than that of the propeller shaft.
In the circuit of FIG. 1, when the speed of the automobile becomes 10 Km/h less than the preset speed, the flip-flop circuit 7 is set, the solenoid 9 is energized to cut the overdrive, and the transmission gears are shifted from fourth into third. Therefore, when, for example, the automobile is running on an uphill grade, the acceleration is increased. Further, when the speed becomes 4 Km/h less than the preset speed, the flip-flop circuit 7 is reset, the overdrive-cutting solenoid 9 is de-energized, and overdrive is resumed.
In the system of FIG. 1, it is possible to switch the automobile from manual to automatic drive by turning on a resume switch (not shown) which initiates operation of the automatic drive control circuit 4, the overdrive-cutting solenoid 9, and so on. On the other hand, switching from automatic drive to manual is automatically effected by operating the brake, the clutch, and so on.
However, the above-mentioned conventional automatic drive system has the following disadvantages:
1. When the automobile is running at a high speed, for example, at a speed greater than 135 Km/h, shifting the gears from fourth into third to keep the speed constant may result in extremely increased engine revolution, giving rise to problems such as an increase in engine temperature and a decrease in the durability of the engine.
2. When the automobile has an automatic transmission and is being driven manually in fourth gear at a speed less than the set point speed, if the driver removes his foot from the accelerator and manipulates the resume switch to cut the overdrive, usually the gears are shifted from fourth to third with the throttle valve being closed. Therefore, the engine brake is applied since the gears are not shifted out of third; i.e., the automobile is first decelerated for a short time and then accelerated, causing a feeling of discomfort on the part of the driver and passengers.
3. When the above-mentioned cutting of the overdrive is effected under certain circumstances, for example, if the automobile is running on a downhill grade, if the output of the internal-combustion engine is large, or if the reactive force of the link coupled to the throttle valve is weak, the degree of acceleration is very great, thereby frightening the driver and passengers.